When attempting to upload a file in the ``Quotation Templates`` without saving the template first, an error occurs in the terminal.

Steps to reproduce:
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- Install the ``sale_management`` module
- Sales > Configuration > Sales Order > Quotation Templates
- Create New > Quote Builder > Click on Upload
- Try to upload a file

Traceback:
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``ValueError: invalid literal for int() with base 10: 'false'``

Previous Behaviour:
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When attempting to upload a file in the ``Quotation Templates`` without first saving the template, the template ID is not generated at [1]. As a result, at [2], we encounter an issue where ``int(sale_order_template_id) = 'false'``. This happens because the template tries to access its name in the uploaded file. If the template name is not available and we attempt to upload the file, ``resId`` at [1] will be returned as false.

Solution:
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Until the template is saved, the upload button will remain hidden. Once the template is saved, the upload button will be displayed. This ensures that when a file is uploaded, the template name will be properly associated with it.

[1]- https://github.com/odoo/odoo/blob/c5ce138697d7c5ebe200566dfb8b7ba55365f100/addons/sale_pdf_quote_builder/static/src/js/quotation_document_kanban/quotation_document_kanban_widget.js#L21-L23

[2]- https://github.com/odoo/odoo/blob/c5ce138697d7c5ebe200566dfb8b7ba55365f100/addons/sale_pdf_quote_builder/controllers/quotation_document.py#L24-L26

sentry-5963406254

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This partially redoes odoo/enterprise#64900. Copying tax_ids from the sale order lines to the down payment line on the invoice doesn't work for external tax calculators. Tax percentages returned by tax calculators are only informational. When they return e.g. 5% tax, the actual amount can be different from 5% because of differences in rounding or (partial) exemptions. The authoritative tax is returned as a value amount (e.g. $10). Because of this, we cannot rely on Odoo to calculate the tax from account.tax records. It's not always going to match.

It's not possible to fix this by externally calculating taxes on the down payment invoice. The single down payment line cannot accomodate all different tax categories. Creating a down payment line per tax category (e.g. avatax_category_id) was considered, but a specific down payment product would need to be created per used tax category which would clutter the database. On top of that, down payment products are removed in later versions [1] which would complicate this approach even further.

This solves the issue by not calculating taxes on down payments at all. The down payment invoice will not be sent to external tax calculators. The resulting down payment lines on the SO won't have taxes set, and those lines will be excluded from the request to the external tax calculator. The final, "regular invoice" transaction will happen as if no down payments were made. The down payment lines won't be sent, so this will look like a regular invoice from the external calculator's view.

By default, the downpayment lines equal the subtotal of the selected down payment amount. We remove tax_ids from the lines because we cannot count on calculating tax locally (see above). To keep the behavior analogous we override the downpayment wizard to create a single down payment line with a total equal to the entered total in the wizard.

[1] odoo/odoo@9aa52dd6418e5881adc2d96d15d062b55d6150c5

opw-3971513